1. Field Of The Invention
The present invention relates to the field of decontamination of nuclear reactor primary systems. More specifically, it relates to a unique method of removing suspended and dissolved solids from chemical decontamination fluids.
2. Description Of The Prior Art
The problem of excessive personnel exposures caused by high background radiation levels in a nuclear reactor primary system, such as in pressurized water reactor (PWR) systems, and the resultant economic cost of requiring personnel rotation to minimize individual exposure is significant at many nuclear plants. These background levels are principally due to the buildup of corrosion products in certain areas of the plant. The buildup of corrosion products exposes workers to high radiation levels during routine maintenance and refueling outages. The long term prognosis is that personnel exposure levels will continue to increase.
As a nuclear power plant operates, the surfaces in the core and primary system corrode. Corrosion products, referred to as crud, are activated by transport of the corroded material to the core region by the reactor coolant system (RCS). Subsequent release of the activated crud and redeposition elsewhere in the system produces radiation fields in piping and components throughout the primary system, thus increasing radiation levels throughout the plant. The activity of the corrosion product deposits is predominately due to Cobalt 58 and Cobalt 60. It is estimated that 80-90% of personnel radiation exposure can be attributed to these elements.
One way of controlling worker exposure, and of dealing with this problematic situation, is to periodically decontaminate the nuclear steam supply system using chemicals, thereby removing a significant fraction of the corrosion product oxide films. Prior techniques had done very little to decontaminate the primary system as a whole, typically focusing only on the heat exchanger (steam generator) channel heads.
Two different chemical processes, referred to as LOMI (developed in England under a joint program by EPRI and the Central Electricity Generating Board) and CAN-DEREM (developed by Atomic Energy of Canada, Ltd.), have been used for small scale decontamination in the past. These processes are multi-step operations, in which various chemicals are injected, recirculated, and then removed by ion-exchange. Although the chemicals are designed to dissolve the corrosion products, some particulates are also generated. One method of chemical decontamination, focusing on the chemistry of decontamination, is disclosed in U.K. Patent Application No. GB 2 085 215 A (Bradbury et al.). There is little disclosure, however, of the methodology to be used in applying that chemistry to system decontamination.
While these chemical processes had typically been used on only a localized basis, use of these chemical processes has now been considered by the inventors herein for possible application on a large scale, full system chemical decontamination. Such an application is disclosed generally in co-pending Application Ser. No. 07/62/120 entitled "System For Chemical Decontamination Of Nuclear, Reactor Primary Systems", and incorporated herein by reference.
While some work has been done in the boiling water reactor (BWR) programs, the BWR scenarios examined by those in the field involved only decontaminating fuel assemblies in sipping cans employing commercial processes at off-normal decontamination process conditions with little regard for the effects of temperature, pressure, and flow that would be mandated by an actual application of the process to the full reactor system.
The estimated collective radiation dose savings over a 10-year period following decontamination is on the order of 3500-4500 man rem, depending upon whether or not the fuel is removed during decontamination. At any reasonable assigning of cost per man-rem, the savings resulting from reduced dose levels will be in the tens of millions of dollars.
As a result of the examination of potential full system decontamination, a need now exists for an effective and economic method to remove dissolved and particulated corrosion products generated by the application of the known chemical decontamination techniques from the chemically-injected primary system fluids.